With an increase of integration density of semiconductor device, an exposing device of a projection type among others tends to use shorter wavelength of exposing light from g line (436 nm) to i line (365 nm) and even up to exciter laser light (248 nm). On the other hand, a numerical aperture (NA) of a reducing lens of the exposing device is also increased, causing the focal depth of the lens to be shorter. Therefore, it is necessary to match a semiconductor wafer surface highly precisely with a focal plane of the lens of the projection exposure device.
An example of a conventional exposing device is shown in FIG. 9 which includes a semiconductor wafer 1 as a sample material, a circuit pattern drawing mask, 2 a mask stage 3 holding the circuit pattern drawing mask 2 and movable in X and Y directions, an illumination light source 4, a reduction lens 5 supported by a frame (not shown) and arranged between the wafer 1 and the circuit pattern drawing mask 2, and a pattern detector 6 for detecting an alignment pattern 1a provided on the wafer 1. Further, in FIG. 9, a stage 7 has the wafer 1 mounted thereon. The stage 7 is movable three dimensionally in X, Y and Z directions. Motors 8a, 8b drive the stage 7 in X and Y directions, and laser distance meters 9a, 9b measure a position of the stage 7 correspondingly to the motors 8a and 8b, with an air micrometer 10 detecting a position of the upper surface of the wafer 1 in Z direction, i.e., height of the upper surface. A stage position control system 11 controls the motors 8a and 8b and the laser distance meters 9a and 9b, and an air micrometer signal processing system 12 is responsive to an output of the air micrometer 10 representing the height of the upper surface of the wafer 1 to supply it to a main control system 13.
In the projection exposure device constructed as mentioned above, the wafer 1 is mounted on the stage 7 with its upper surface being adjusted to a focal plane of a reduction lens 5. Then, any deviation of an alignment pattern 1a on the wafer 1 with respect to a pattern of a circuit pattern drawing mask 2 which is projected on the upper surface of the wafer 1, which may be caused by a possible deformation of the lens or possible misalignment thereof in an assembly, detected by a pattern detector 6 and the stage is moved by moving the stage 7 by actuating the stage control system 11 such that an amount of the detected deviation becomes zero, completing a positional alignment between the wafer 1 and the circuit pattern drawing mask 2. Thereafter, an exposure is performed by opening a shutter of an illumination source 4.
The positional alignment between an actually focused plane of the reducing lens 5 and the upper surface of the wafer 1 is performed by detecting the height of the upper surface of the wafer 1 by supplying an output of the air micrometer 10 to the micrometer signal processing system 13, moving the stage 7 in Z direction by the stage control system 11. The signal processing system 13 may be constructed as disclosed in, for example, U.S. Pat. No. 4,115,762, assigned to the assignee of this application.
In this system, however, since the focal plane of the reduction lens 5 varies in dependence upon (i) atmospheric pressure and temperature, (ii) temperature variation of a support structure of the reduction lens 5 and (iii) drift of the air micrometer 10, etc., and it is substantially impossible to obtain an exact alignment of the upper surface of the wafer 1 with respect to the actual focal plane of the reduction lens 5 by a single adjustment, it has been usual that a plurality of wafers 1 are exposed as trial and any of these wafers which exhibits a satisfactory exposure is selected to obtain the actual focused position. The height of the upper surface of the wafer 1 is controlled by using an output value of the air micrometer 10 corresponding to the thus obtained actual focal plane as a reference and an actual exposure is performed.
On the other hand, as the exposing device for projecting a desired pattern such as reticle onto an object such as semiconductor wafer, Japanese Kokai (P) 57-212406 discloses a technique for focusing automatically and quickly with high precision, in which a minute light transparent portion is formed in a portion of the pattern to be projected onto the object and the light transparent portion is projected on the object. Reflection from the projected minute portion on the object is focused on a plane of the pattern and passed through the transparent portion. An amount of reflected light passed through the portion is detected.
In the above conventional exposing apparatus, an actual focus position on the wafer 1 is obtained by repeating a test exposure and an actual exposure is performed by controlling the height of the upper surface of the wafer 1 on the basis of an output value of the air micrometer 10 at the thus obtained focused position. Therefore, it takes at least several tens of minutes to perform the repetitive test exposures and thus it is impossible to compensate for drift produced in the apparatus, etc., within a short time in the order of several minutes, resulting in a degraded output due to the test exposures. Further, since the focal depth of the reduction lens 5 becomes smaller than that of the conventional lens, any undulation or bending of the surface of the wafer 1 due to variation of thickness thereof, etc., may produce a tilting of wafer surface with respect to an optical axis within an exposing area thereof. This represents a very important problem. In order to detect such tilting of wafer surface, it becomes popular to not detect an average height of the wafer surface by the air micrometer 10 but measure a height and inclination of the wafer surface within the exposing area thereof by a suitable level meter or tiltmeter. In order to perform an actual exposure, however, it is necessary to set reference values in the level meter, which reference values are values of the height and the inclination. In order to set these reference values, it is necessary to detect height and inclination of an actual focal plane of the reduction lens 5 and to align the wafer 1 or other flat plane to be used as reference to the detected focal plane position.
In the aforementioned Kokai 57-212406, the detection of the amount of reflected light passed through the fine transparent portion is performed by sequentially detecting maximum light amount of respective points on the surface of the wafer to be exposed. That is, this technique does not detect maximum light amounts at a plurality of points simultaneously. Therefore, although it is possible to adjust the surface of the wafer at the optimum focal position, it is impossible previously to detect the height and inclination of the surface within the exposing area thereof and thus it is impossible to precisely obtain a focussed flat plane.